Pipettes used for liquid dosage in laboratories comprise a piston movable in a cylinder for aspiration of liquid into a tip container connected with the cylinder. The volume is usually adjustable. There are also electronic pipettes whose piston is actuated by means of an electric motor and a control system associated with it. However, there are also electronic pipettes whose pistons are actuated by manual force and which comprise an electronic display only. Electronic pipettes have a user interface for selection of the desired pipette function (e.g., forward or reverse pipetting), setting of the volume and for giving commands for performing operations. The user interface has the necessary switches for input of the necessary settings and performance of the functions. The user interface is connected to a display, by means of which the volume and other necessary data can be displayed. The display can also show menus allowing data input in the control system.
Examples of known electronic pipettes are disclosed in International Publication Nos. WO 2005079989 and WO 2005079987, for example.
Pipettes usually have a calibration function allowing the piston stroke or the volume indicated on the display to be set so that the dosed liquid volume equals the indicated volume with maximal accuracy. In practice, calibration comprises weighing the mass of the liquid amount dosed by the pipette with an indicated volume. The liquid is usually distilled water and the calibration is performed at room temperature (20-25° C.). Usually, weighing is repeatedly carried out and the mean value of the results of the weighing operations is calculated.
However, due to the use of pipette during its lifetime, the parts of the pipette experience mechanical wear causing play and backlash between different components of the pipette, which affect the actual volumes of liquid aspired and/or dispensed with the pipette.
U.S. Pat. No. 5,187,990 discloses an electronic pipette that automatically goes through calibration process upon either powerup or restoration of power after power loss. In the calibration process, the digital linear actuator of the pipette undergoes full extension in order to properly position the piston of the pipette in its home position prior to starting the pipetting processes. For compensating air pressure and liquid surface tension, among others, the piston of the pipette is moved for additional distance during the aspiration and/or dispensing of a volume of liquid.